Remove unused custom yui/calendar.css and yui/sprite.png

They were removed for 4.0 in upstream in
https://bugzilla.mozilla.org/show_bug.cgi?id=572949#c8

CSS is loaded from /js/yui/assets/skins/sam/calendar.css
anyway.

Bug: 54823
Change-Id: Ia4c5196362adb7318dc9b180be46512c63f0edb4
Merge "Remove unused custom yui/calendar.css and yui/sprite.png"